1. Performance of Recommender Algorithms on Top-N Recommendation Tasks
Gabriel Vargas Carmona

2. Agenda Introduction Evaluation Long-tail Collaborative algorithms
General Overview
Recommender system
Evaluation
RMSE & MAE
Recall and precision
Long-tail
Netflix and Movielens
Collaborative algorithms
Neighborhood models
NNCosNgbr
Latent factor
PureSVD
Case of Study
Results
Conslusion

3. General Overview Few years ago… …Nowadays More information
Information was limited

4. Recommender system
The first systems appear at the beginning of the 90´s
Is typically based in a set of users and a set of items.
It works when each user “A” rates a subset items with some numeric value. The recommender system has to predict the unknown rating for user “A” on a non-rated target item “Y” based on the known ratings.
There is a very large number of items and the user is not aware of them, the system suggests a few specific items that can be appealing to him.
Start users, cold start.

5. Non-personalized models
Rating without depending on the user.
Non personalized algorithms can be compared with personalized algorithms.
Algorithms as baselines.

6. Recommender system
User A
User B

7. RMSE & MAE Most known error methods
Evaluation
RMSE & MAE
Most known error methods
Recommender systems are evaluated with error metrics such as RMSE (actual raitings vs raitings predicted by the system).
These methods do not measure the top-N performance.
Sometimes commercial systems present the “best bet”, without taking into consideration the predicted rating values.

8. Recall and precision Performance analysis measurement.
Precision is the fraction of retrieved instances that are relevant, while recall is the fraction of relevant instances that are retrieved.
Example:
Supose a search engine return 60 pages.
Only 30 are relevant.
Failed to return 40 additional relevant pages.
Precision can be understand as 40/60 while its recall is 30/70.

9. Long-tail
Movielens and Netflix
Long-tail is applied to the distribution of rated items in a comercial system. Majority of ratings are condensed in a small fraction.

10. Collaborative filtering
Collaborative algorithms
Collaborative filtering
Most of the recommender systems are based on collaborative filtering (CF). Recommendations are based on past user behavior.
Relation between user to users, items to items and finally users to items.
Two type of approaches: neighborhood models and Latent factor.

11. Neighborhood models
This models base their prediction on the similarity among users or items. They represent the most common approach to the CF.
Two types of algorithms:
centered on user-user, predict the rating based on ratings by similar users
centered on item-item, predict the preference for an item based in similar items.
The neighborhood model is from the item-item algorithm.
Sparse dataset in some cases. A coefficient for shrinkage is defined.

12. Neighborhood models
Neighborhood models are improved by means of KNN (k-nearest-neighborhood) approach. It decreases noise and improves the quality recommendations.
Here are only considered the k items rated by “A” that are most similar to “Y”. This method also considers the biases.

13. Neighborhood models
Considering that for top-N recommendation task an exact rating is not needed, items are rank simply by their appeal to the user. The formula is simplified.
It is important to mention that does not represent a proper rating, but is rather a metric for the association between user “A” and it “Y”

14. Latent factor models
They are formally known as the SVD models standing for Singular Value Descomposition. This type of models approaches model users and items as vectors. They have the use of matrix, and in the same space users and items are comparable; the rating of user “A” on item “Y” is predicted by the proximity between the related latent factor vectors.
The idea of the SVD models is to factorize the user- item rating matrix to a product of two lower rank matrices, user factor and item factor. Moreover, each user “A” is represented with a user actor vector . Similarly, each item “Y” is represented with an item factor vector . Prediction of a rating given by user “A” for item “Y” is computed as the product adjusted for biases

15. Results
According to the case of study explained, the quality of the datasets for MovieLens and Netflix are presented.
MovieLens

16. Results
Netflix

17. Conclusions
The way the recommender can be analyzed is based into accuracy metrics and error metrics.
Top-n recommendations are really useful when managing the marketing of products that are not known by the people.
The collaborative algorithm is the best way to understand the relation between items, users and both together.
To make an evaluation with higher accuracy we need to consider the top rated items and the bias they represent.
We have to consider also that the results given are only analyzed for this article, in order to have a more objective data more measurements should be made.

18. References
[1] P. Cremonesi, Y. Koren and R. Turrin. Performance of Recommender Algorithms on Top-N Recommendation Tasks. Page consulted on 15 June Available at: der%20algorithms%20on%20top- n%20recommendation%20tasks&source=web&cd=1&ved=0CE4QFjAA&ur l=http%3A%2F%2Fwww.research.yahoo.net%2Ffiles%2Frecsys2010_submis sion_150.pdf&ei=tqnjT5nbIYjUsga6h- DFCQ&usg=AFQjCNFiOt8A6RYLMPYJ_02k2oWeYHhBwA
[2] S. M. Galán. Filtrado Colaborativo y Sistemas de Recomendación. Page consulted on 15 June Available at:
[3] M. Jamalí and M. Ester. Using a Trust Network to Improve Top-N Recommendation. Page consulted on 17 June Available at: TopNRecommendation.final.pdf
[4] E notes. Precision and recall. Page consulted on 17 June Available at:
Statsoft. K-Nearest neighbors. Page consulted on 17 June Available at: